Code example:

    public static void main(String[] args) throws IOException {
    FileWriter writer = new FileWriter("src/main/resources/SomeText.txt", true);

    writer.append("");
    writer.append("");
    writer.write("File");
    writer.write("Writer");
    writer.append("");
    writer.write("Test");

    writer.close();
    }

Textfile example:

| Textfile | Textfile I want | Textfile I get |
|:--------:|:---------------:|:--------------:|
|One       |One              |One
|Two       |Two              |Two
|Three     |File             |Three
|Four      |Writer           |Four
|Five      |Five             |FiveFileWriterTest
           |Test             |

I read that you should use new FileWriter("text.txt", true) but that doesn't work.

I want to skip some lines that I don't need to write.

CodePudding user response：

First clarify the different I/O classes for a file:

1. your given FileWriter writes to a file, i.e. equivalent to appends
2. in contrast - a FileReader reads from an existing file

As far as I know there is no such thing as a FileUpdater in standard Java packages which can read lines from a file an skip some while overwriting others.

But the concept of update, replace or skip lines can be implemented by combining both classes Reader and Writer.

See also GeeksforGeeks: File handling in Java using FileWriter and FileReader

Write to a text-file from an array of lines

Similar to what given code already didL

FileWriter writer = new FileWriter("src/main/resources/SomeText.txt", true);

List<String> lines = List.of("1", "2", "3", "4", "5");

for (line in lines) {
    writer.append(line);  // equivalent with writer.write(line)
    writer.append("\n");  // add a newline
}
writer.close();  // this will flush and write from buffer to file

Now you should have 5 new lines in the file.

Read your given text-file into an array of lines

Suppose you want to read from a given text-file src/main/resources/SomeText.txt. You can use FileReader to read char by char or a special class which buffers the chars and allows to read entire lines: BufferedReader and its method readLine():

BufferedReader reader = new BufferedReader(new FileReader("src/main/resources/SomeText.txt"));

List<String> lines = new ArrayList<String>();  // empty array list

while (reader.ready()) {  // while not End of File (EOF)
    lines.add(reader.readLine());  // read next line and add to the list
}
reader.close();  // this will close the file

// count the lines and print to console
System.out.println(lines.size());

Count of read lines should be 5 if the file was created freshly before. Otherwise if the file existed with lines before, the line-count should be 5 more than before appending/writing the new ones.

Insert lines

Now you can combine both recipes to read a given file and add/insert new lines where needed.

CodePudding user response：

Just change writer.append(""); to writer.write("");

A line that is skipped, is the same as a line that has nothing written